The terms thin films or thin layers, refer to layers in the micrometer (μm) and/or nanometer (nm) thickness range. The manufacturing of thin layers is relevant in industrial production processes, for example to apply functional layers with fine-tuned properties while saving expensive raw materials. Such thin films have a variety of applications in optics, microelectronics and the treatment of surfaces. A uniform thickness and well-defined characterization of the layer is a challenge for the manufacturer.
For example, to improve the corrosion resistance and the adhesion of lacquers, seals and adhesives on aluminum strips, a conversion coating may be applied through a process called coil-coating. Previously, these conversion coatings contained chemical components with chromium as an ingredient; however, industry is moving to use chromium-free based conversion coatings. During production it may be necessary to monitor the quality of the applied conversion coating and/or their chemical composition, in particular the amount of key ingredients in real-time. However, in a coil-coating process, the aluminum strip may move with a speed up to several hundred meters per minute through the production machinery.
Certain techniques are known to analyze these kinds of thin layers. All of them share the problem that they are not able to analyze fast-moving samples and nanometer thick layers which are applied on rough surfaces. White light interferometry requires at least a film thickness that is within the range of the wavelength of visible light while the normal thickness of a conversion coating is below 100 nanometers. Photometry also requires thicker layers to obtain the desired sensitivity. X-Ray-fluorescence (online-XRF) may be too slow and, like Beta Backscatter, requires radiation shielding that would be costly in an industrial environment. Since the roughness of a typical aluminum strip surface is in the micrometer range, it is difficult to use ellipsometry, which requires very flat surfaces like in the semiconductor industry. During the coil-coating process, the aluminum strip will move fast and vibrate, which rules out attenuated total reflection (ATR) spectroscopy, which needs a distance to the samples surface smaller than the wavelength used.
Further, the photo-acoustic (hereinafter also referred to as PA) techniques based on the PA principle are known to measure film layers in which a sample is exposed to electromagnetic radiation. The absorption of the radiation leads to a higher temperature in the sample and volume change, which is followed by a dilation of the sample surface. In-turn, the surface dilation causes an impulse or periodic changes of the surrounding medium density, which may be detected with a microphone as sound. The sensitivity to sample ingredients using the photo-acoustic technique may be better than conventional light based spectroscopy. However, in known PA techniques, the sample thickness has not been smaller than about 12 micrometers and the microphone is placed in mechanical contact with (i.e. touching) the sample or requires a liquid medium in contact with the sample to transmit sound to the microphone, which may be unsuitable for measuring conversion coatings below 100 nanometers in the coil-coating manufacturing process.
Therefore, a fast, real-time, nondestructive and non-mechanical-contact measurement technique for conversion layers that is insensitive to noise, dirt and shocks suitable for use in the coil coating manufacturing process is desired.